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Paleontologists recently added a new step to the formerly four-step hypothesis on the evolution of vertebrates, which include human beings, after determining that fossilized ancient worms found in southwest China's Yunnan Province represented the "first step" of the evolutionary process. The new hypothesis, raised by Chinese paleontologist Shu Degan in partnership with a group of scholars from Britain and Japan, was revealed in the US-based Science magazine earlier this year. The theory depicted in the prestigious scientific journal outlined the evolutionary process of deuterostomes, one of the basic two pedigrees on the "crown" of the fauna's evolutionary tree, and concluded that vertebrates originated in at least five steps. According to the four-step theory, vertebrates on the top of the deuterostomes pedigree had no relationship with protostomes, and vertebrates stemmed from echinoderms, which were the lowest deuterostomes, and hemichordates. "This hypothesis is based on modern zoological information," said Professor Shu Degan, who works with Northwest China University in Xi'an, capital city of northwest China's Shaanxi Province. "Whether the hypothesis is reliable or not should be tested, revised and reinforced with authentic historic data," Shu added. Shu believed that effective research on the origins of vertebrates in paleological terms should search for fossil evidences in line with clues provided by the modern zoological information. Such evidence was discovered at Chengjiang Lagersttat, a county known for its bountiful wealth of fauna fossils in Yunnan Province. The evidence included fossilized jawless Kunming and Haikou fishes, or Myllolunimingia and Haikouichthys respectively in scientific names, which were believed to be the most primitive vertebrates living 530 million years ago, and ancient worms and Yunnan worms, or vetulicolians and yunnanozoans respectively, which reportedly lived in the same geologic age as the Haikou fishor the previous period. "The serial finds of the early deuterostomes not only add evidence to the four-step theory but also help add the 'first step' to the evolutionary process of the vertebrates," Professor Shu said. According to Shu, the ancient worms and Yunnan worms, which are believed to be the origins of deuterostomes, and thus the "first step" of the evolutionary process of vertebrates, might be regarded as real deuterostomes in terms of such metabolic forms as food absorbing and breathing, although their bodies retained some characteristics of their ancestors. Based on comparative anatomical research on more than 1,400 samples of fossilized Haikou worms, or Haikouella, paleontologists found that the "first step" fauna not only lacked notochord structures but also differed fundamentally from chordates in skin, muscle, respiratory and nerve organs. Though the new "five-step" hypothesis on vertebrate origins was raised on the basis of the latest research results of modern zoology and of the fossil series of the most primitive deuterostomes in China, the theory has only outlined briefly the evolutionary process, and some intermediate links are still lacking, Professor Shu said. "There is still a long way to go before we can determine a detailed chronology of the evolutionary process of vertebrates," Shu added.